Shedding a new light on Huntington's disease: how blood can both propagate and ameliorate disease pathology.

Huntington's disease (HD) is a monogenic neurodegenerative disorder resulting from a mutation in the huntingtin gene. This leads to the expression of the mutant huntingtin protein (mHTT) which provokes pathological changes in both the central nervous system (CNS) and periphery. Accumulating evidence suggests that mHTT can spread between cells of the CNS but here, we explored the possibility that mHTT could also propagate and cause pathology via the bloodstream. For this, we used a parabiosis approach to join the circulatory systems of wild-type (WT) and zQ175 mice. After surgery, we observed mHTT in the plasma and circulating blood cells of WT mice and post-mortem analyses revealed the presence of mHTT aggregates in several organs including the liver, kidney, muscle and brain. The presence of mHTT in the brain was accompanied by vascular abnormalities, such as a reduction of Collagen IV signal intensity and altered vessel diameter in the striatum, and changes in expression of Glutamic acid decarboxylase 65/67 (GAD65-67) in the cortex. Conversely, we measured reduced pathology in zQ175 mice by decreased mitochondrial impairments in peripheral organs, restored vessel diameter in the cortex and improved expression of Dopamine- and cAMP-regulated phosphoprotein 32 (DARPP32) in striatal neurons. Collectively, these results demonstrate that circulating mHTT can disseminate disease, but importantly, that healthy blood can dilute pathology. These findings have significant implications for the development of therapies in HD.

Use of adeno-associated virus-mediated delivery of mutant huntingtin to study the spreading capacity of the protein in mice and non-human primates.

In order to model various aspects of Huntington's disease (HD) pathology, in particular protein spread, we administered adeno-associated virus (AAV) expressing green fluorescent protein (GFP) or GFP coupled to HTT-Exon1 (19Q or 103Q) to the central nervous system of adult wild-type (WT) mice and non-human primates. All animals underwent behavioral testing and post-mortem analyses to determine the long-term consequences of AAV injection. Both mice and non-human primates demonstrated behavioral changes at 2-3 weeks post-surgery. In mice, these changes were absent after 3 months while in non-human primates, they persisted in the majority of tested animals. Post-mortem analysis revealed that spreading of the aggregates was limited, although the virus did spread between synaptically-connected brain regions. Despite circumscribed spreading, the presence of mHTT generated changes in endogenous huntingtin (HTT) levels in both models. Together, these results suggest that viral expression of mHTTExon1 can induce spreading and seeding of HTT in both mice and non-human primates.

Demonstration of prion-like properties of mutant huntingtin fibrils in both in vitro and in vivo paradigms.

In recent years, evidence has accumulated to suggest that mutant huntingtin protein (mHTT) can spread into healthy tissue in a prion-like fashion. This theory, however, remains controversial. To fully address this concept and to understand the possible consequences of mHTT spreading to Huntington's disease pathology, we investigated the effects of exogenous human fibrillar mHTT (Q48) and huntingtin (HTT) (Q25) N-terminal fragments in three cellular models and three distinct animal paradigms. For in vitro experiments, human neuronal cells [induced pluripotent stem cell-derived GABA neurons (iGABA) and (SH-SY5Y)] as well as human THP1-derived macrophages, were incubated with recombinant mHTT fibrils. Recombinant mHTT and HTT fibrils were taken up by all cell types, inducing cell morphology changes and death. Variations in HTT aggregation were further observed following incubation with fibrils in both THP1 and SH-SY5Y cells. For in vivo experiments, adult wild-type (WT) mice received a unilateral intracerebral cortical injection and R6/2 and WT pups were administered fibrils via bilateral intraventricular injections. In both protocols, the injection of Q48 fibrils resulted in cognitive deficits and increased anxiety-like behavior. Post-mortem analysis of adult WT mice indicated that most fibrils had been degraded/cleared from the brain by 14 months post-surgery. Despite the absence of fibrils at these later time points, a change in the staining pattern of endogenous HTT was detected. A similar change was revealed in post-mortem analysis of the R6/2 mice. These effects were specific to central administration of fibrils, as mice receiving intravenous injections were not characterized by behavioral changes. In fact, peripheral administration resulted in an immune response mounting against the fibrils. Together, the in vitro and in vivo data indicate that exogenously administered mHTT is capable of both causing and exacerbating disease pathology.

Mutant huntingtin protein expression and blood-spinal cord barrier dysfunction in huntington disease.

OBJECTIVE: The aim of the study was to assess the distribution, frequency, and specific location of mutant huntingtin protein (mHTT) aggregates-the pathological hallmark of Huntington disease (HD)-within the various compartments of the spinal cord and their potential impact on the local vasculature and blood-spinal cord barrier (BSCB). METHODS: We performed a series of postmortem immunohistochemical and immunofluorescent stainings, as well as Western blot analyses, on cervical and lumbar sections of the spinal cord in patients diagnosed with HD (n = 11 of all grades of disease severity) along with sex- and age-matched healthy controls (n = 9). RESULTS: We observed that mHTT was preferably expressed within the anterior horn of the gray matter, in both cervical and lumbar sections. At the cellular level, mHTT aggregates were more often encountered in the extracellular matrix but could also be observed within cell bodies and neurites as well as within the endothelium of blood vessels with an increase in the density of small blood vessels in cervical sections of HD cases. These vasculature changes were accompanied with features of BSCB leakage, as assessed by the presence of increased levels of fibrinogen in the surrounding parenchyma and enhanced leukocyte infiltration. INTERPRETATION: This alteration in BSCB integrity may be explained, in part, by the dysregulation we found in some of the main proteins associated with it such as junctional adhesion molecule-1 and vascular endothelial cadherin. These observations have important implications for our understanding of HD pathology and may also have significant therapeutic implications. Ann Neurol 2017;82:981-994.